The combustion of coal for the generation of electricity in power plants results in production of large quantities of waste or by-products such as fly ash, bottom ash, and flue gas desulfurization by-products (FGD-BPs). In flue gas desulfurization, large quantities of calcium sulfites or calcium sulfates are produced such as by reaction of lime or limestone with the sulfur dioxide in desulfurization units. The vast majority of such flue gas desulfurization by-products are disposed of in landfills or surface impoundments. Interest in using such by-products on agricultural land, especially as an acid soil amendment, has risen.
With the use of lime or limestone for desulfurization of flue gases, calcium sulfite, calcium sulfate (gypsum: CaSO.sub.4.2H.sub.2 O) or mixtures thereof are normally produced, and the use of gypsum as a soil additive is known. High-sulfate gypsum by-products can enhance plant growth, as reported in Phosphogypsum in Agriculture: A Review, Alcordo, I. S. and Rechcigl, J. E.; Adv. Agron. 49:55-118 (1993). The applications of fluidized bed combustion by-products (FBC-BPs) to agricultural land and long-term effects and soil chemical properties of gypsiferous FBC-BPs have also been reported by Korcak, R. F., and Kemper, W. D.; "Long-term effects of gypsiferous coal combustion ash applied at disposal levels on soil chemical properties," Plant and Soil 154:29-32 (1993) and by Shainberg, I., Sumner, M. E., Miller, W. P., Farina, M. P. W., Payan, M. A., and Fey, M.V.; "Use of Gypsum on Soils: A Review", in Advances in Soil Sciences, Vol. 9 Springer-Verlag, New York (1989) at pages 1-111. Several studies have shown no effect of gypsum during the first year of application in field experiments even when applied at relatively high rates. Although some studies have shown enhanced growth due to calcium and sulfur, enhanced gypsum effects often occurred during droughty years which might indicate improved subsoil physical/chemical properties which improve plant access to subsurface water and nutrients. Plants grown on soils with especially low or marginal soil calcium might respond more extensively to calcium as a nutrient, and gypsum might be expected to enhance plant growth at any level of application, as described in "Calcium Deficiency in Clayey Horizons of Savanna Oxisols", by Ritchey, K. D., Silva, J. E., and Costa, V. F., Soil Sci., 133: 378-382 (1982). Sources of gypsum used in most studies have been mined gypsum or phosphogypsum by-products derived from the phosphate fertilizer industry, and not coal combustion by-products. Gypsum contains a high sulfur content and phosphogypsum contains phosphorus, so sulfur and phosphorus as nutrients should not be ruled out as reasons for plant enhancement by these products.
Growth of weeping lovegrass [Eragrostis curvula (Schrader) Nees] and lespedeza [Lespedeza cuneata (Dum.-Cours.) G. Don] was very poor when plants were grown with unleached 100% FGD-BPs as described by Shahandeh, H. and Sumner, M. E. in "Establishment of Vegetation on By-product Gypsum Materials"; J, Environ. Qual.22:57-61 (1993). Growth reductions were attributed to ion toxicity (probably B and/or Cl), salinity (high electrical conductivity, EC), and poor hydraulic properties (poor texture from high levels of silt-sized particles) of the FGD-BPs used. Growth was reduced unless these FGD-BPs were leached and mineral nutrients added, especially N, P, and K; N being the most limiting of the nutrients. Differences among FGD-BPs were also noted for root penetration and distribution. In other studies, alfalfa (Medicago sativa L.) and tall fescue (Festuca arundinacea Schreber) yields were increased by FGD amendment, although in the earlier harvests, yields were suppressed at the higher amendment rate, as reported by Sutton, P. and Stehouwer, R. C., in "Use of Flue Gas Desulfurization By-products as ag-lime substitutes"; In Agron. Abstr, p. 293, Am. Soc. Agron, 1992. Alfalfa yields were reduced more than tall fescue at the rates used (0.35 to 2.8% of soil mixes). Yields of later cuttings (3rd and 6th) of the plants grown with these coal combustion by-products (CCBPs) were similar. FGD-BPs added to acid coal mine spoil (overburden) and underclay (subsoil) at different rates (up to 24% of soil mix) reduced tall fescue growth, but yield of later cuttings was enhanced (Stehouwer, R. C., Sutton, P., and Dick, W. A., (1993). Fescue growth on acid mine spoil amended with FGD and sewage sludge. p. 50. In Agron. Abstr., Am. SOc, Agron., Madison, Wis.). No unusual toxicity symptoms were noted on leaves, but leaf B for plants grown with one CCBP was sufficiently high to suspect potential B toxicity if certain plants were grown under similar conditions.
A flue gas desulfurization scrubber sludge impoundment was successfully vegetated with four of seven herbaceous [tall wheatgrass (Agropyron elongatum (Host) Beauv.), tall fescue, yellow sweet clover (Melilotus officinalis Lam.), and Japanese millet (Echinochloa crusgalli(L.) Beauv.)] and two of six tree [eastern cottonwood (Populus deltoides Marsh.) and eastern red cedar (Juniperus Virginiana L.] species tested when fertilizer (N, P, and K) and other amendments (cow manure for herbaceous plants and woodchips for trees) were added as reported by Mulhern, D. W., Robel, R. J., Furness, J. C., and Hensley, D. L. in "Vegetation of Waste Disposal Areas at Coal-Fired Power Plant in Kansas; J. Environ. Qual. 18:285-292 (1989). The treated plots had greater productivity than untreated plots seven years after the initial application of the products as reported by Wilson, G. W. T., Hetrich, B.A.D., and Schwab, A. P. in "Reclamation Effects on Mycorrhizae and Productive Capacity of Flue Gas Desulfurization Sludge" J. Eviron. Qual.20, 777-783 (1991). The greater productivity on the treated plots was attributed, in part, to mycorrhizal associations with roots.
We have now found that flue gas desulfurization by-products of primarily gypsum, which contain between one to about twenty percent magnesium hydroxide are, quite surprisingly, excellently suited for use as soil ameliorants to enhance plant growth.